1. Field of the Invention
The present invention relates to an optical scanning apparatus and a color image forming apparatus using the same. For example, the present invention is suitable for a color image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multi-function printer, which has an electrophotographic process.
2. Description of the Related Art
Up to now, an optical scanning apparatus is used for a laser beam printer (LBP), a digital copying machine, a multi-function printer, or the like. The optical scanning apparatus is used to scan light onto at least one photosensitive drum. The scanning creates a latent image in the form of a charge distribution left on the photosensitive drum. This charge distribution is subsequently used to complete the laser print. In the optical scanning apparatus, a light beam which is optically modulated according to an image signal in light source unit and emitted therefrom is periodically deflected by an optical deflector composed of, for example, a rotary polygon mirror (polygon mirror). The deflected light beam is condensed in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system having an fθ characteristic. The surface of the photosensitive recording medium is scanned with the light beam to perform image recording.
FIG. 18 is a principal part schematic view illustrating an optical scanning optical system of a conventional optical scanning apparatus.
In FIG. 18, a single or a plurality of diverged light beams emitted from a light source unit 201 are converted into a parallel light beam by a collimator lens 203. The light beam is limited by a stop 202 and then incident on a cylindrical lens 204 having a finite refractive power only in a sub-scanning direction. Of the parallel light beam incident on the cylindrical lens 204, a light beam in a main scanning direction is exited therefrom without change. In addition, a light beam in the sub-scanning direction is condensed and imaged on a deflecting surface 205a of an optical deflector 205 including a polygon mirror to form a linear image.
The light beam deflected by the deflecting surface 205a of the optical deflector 205 is guided onto a photosensitive drum surface 208 which is a surface to be scanned through an imaging lens 206 having the fθ characteristic. The optical deflector 205 is rotated in a direction indicated by an arrow “A”. Therefore, the photosensitive drum surface 208 is scanned with the single or the plurality of light beams in a direction indicated by an arrow “B” (main scanning direction), to perform image information recording.
Various multi-beam optical scanning apparatus for scanning a plurality of light beams to form an image have been proposed as this type of optical scanning apparatus (see Japanese Patent application Laid-Open No. 2003-182139).
In the multi-beam optical scanning apparatus, streak-shaped density unevenness occurs in an interface region between a scanning line of a plurality of light beams scanned by first scanning and a scanning line of a plurality of light beam scanned by second scanning. According to Japanese Patent Application Laid-Open NO. 2003-182139, in order to reduce the density unevenness, an amount of light beams with which a circumference of the interface region are scanned is reduced to be smaller than an amount of light beams with which the vicinity of a center of the plurality of scanning lines are scanned.
The optical scanning apparatus according to Japanese Patent Application Laid-Open No. 2003-182139 includes respective elements necessary to reduce the streak-shaped density unevenness with respect to a single color.
For example, when a large deviation of imaging magnification of an imaging optical system in the sub-scanning direction (sub-scanning cross section) occurs or when a large deviation of an adjusted pitch interval of the scanning lines in the sub-scanning direction occurs, a certain amount of reduction effect is recognized.
However, it is difficult to completely remove the streak-shaped density unevenness.
Even if the density unevenness with respect to the single color does not become a problem, for example, when four colors of cyan (C), magenta (M), yellow (Y), and black (Bk) are superimposed on one another, the density unevenness becomes a large problem.
FIG. 19 is an explanatory diagram illustrating a state of a plurality of scanning lines which occurs in a case where a sub-scanning magnification of the imaging optical system in the circumference of an image is larger than that in the center of the image.
When a scanning line interval in the sub-scanning direction is set to a scanning line interval in the center of the image, which is determined based on a resolution of the optical scanning apparatus, scanning lines are overlapped with each other in edges of the image to increase a density in only a superimposed region in some cases.
FIG. 20 is an explanatory diagram illustrating a state of a plurality of scanning lines which occurs in a case where the sub-scanning magnification of the imaging optical system in the circumference of the image is smaller than that in the center of the image.
When the scanning line interval in the sub-scanning direction is set to the scanning line interval in the center of the image, which is determined based on the resolution of the optical scanning apparatus, scanning lines are separated from each other by one or more line spaces in the edges of the image to reduce a density of only a separated region in some cases.
The above-mentioned cases notably occur when the number of beams (scanning light beams) increases. For example, assume that the sub-scanning magnification in the circumference of the image is deviated from that in the center of the image by 5%. Here, when two-beam scanning is performed, a shift of 0.05 (=(2−1)×5/100) pixels occurs. When four-beam scanning is performed, a shift of 0.15 (=(4−1)×5/100) pixels occurs.
When eight-beam scanning is performed, a shift of 0.35 (=(8−1)×5/100) pixels occurs. When 32-beam scanning is performed, a shift of 1.55 (=(32−1)×5/100) pixels occurs. Therefore, a shift amount increases.
When the two-beam scanning is performed at a resolution of 600 dpi, a number of occurrences of the streak-shaped density unevenness are 11.8 per 1 mm. When the four-beam scanning is performed at the resolution of 600 dpi, the number of occurrence of the streak-shaped density unevenness is 5.9 per 1 mm.
As described above, a spatial frequency of the streak-shaped density unevenness becomes smaller as the number of beams increases, so the streak-shaped density unevenness is more easily visually recognized by human eyes. Therefore, up to now, when the two-beam scanning is performed, no problem occurs because a deviation amount is small and the spatial frequency is within a high frequency band which is difficult to be visually recognized by human eyes. However, the above-mentioned density unevenness becomes a problem in a case of the multi-beam optical scanning apparatus using four or more beams.
In an optical scanning apparatus using, as the light source unit, a multi-beam semiconductor laser including a plurality of light source portions arranged in one dimension, the scanning line interval in the sub-scanning direction is adjusted by rotational adjustment about an optical axis of each of the light source portions. When the imaging magnification of the entire system in the sub-scanning direction is five times and an interval between the adjacent light source portions is 100 μm, the light source portions may be tilted relative to the main scanning direction at an angle θ (=2.42625°) as illustrated in FIG. 21 in the case of the optical scanning apparatus of 1200 dpi.
In an actual case, the adjustment may be performed at an angle deviated from a designed tilt angle by an angle α. For example, as illustrated in FIG. 22, when the light source portions are tilted at θ+α (=2.92625°) because of α (=0.5°), the scanning line interval in the sub-scanning direction becomes 25.53 μm (that is, 1.2 lines). Therefore, even when a mechanism for adjusting the scanning line interval in the sub-scanning direction is provided, it is difficult to completely remove the streak-shaped density unevenness in the interface region between adjacent scanning lines. In even such a case, it is obvious that a problem occurs when the number of beam increases.